An organic EL element for use in an organic electro-luminescence (EL) panel can be produced by forming a layer of an organic EL material (organic EL layer) on a light-transmitting substrate.
The organic EL panel has low extraction efficiency of light emitted from the organic EL layer. There are some causes for the problem. The first cause is that light emitted from the organic EL layer has no directivity and is dissipated in all directions and thus extracted to the front side of the organic EL panel in small amount. The second cause is that the emitted light is totally reflected at the interfaces between the layers in the organic EL panel and thus is not extracted to the outside of the organic EL panel.
The cross-sectional view of FIG. 11 schematically illustrates an example of the problem and the causes. Hatches are not shown in the cross-sectional view of FIG. 11 for visibility. As shown in FIG. 11, the organic EL panel of FIG. 11 includes: a transparent substrate 11; and a transparent electrode (anode) 14; an organic EL layer 16; and a cathode 15 laminated on one surface of the transparent substrate 11 in this order. The transparent substrate 11 is formed of, for example, glass. The transparent electrode 14 is formed of, for example, Indium-Tin Oxide (ITO). The cathode 15 is formed of, for example, a metal. The light emitted from the organic EL layer 16 includes emitted light beams 101 and 102 in the direction orthogonal to the organic EL panel (the direction orthogonal to the transparent substrate 11) or a direction close thereto and emitted light beams 103 and 104 in the direction largely tilted with respect to the orthogonal direction. The emitted light beams 101 and 102 are not totally reflected at the interfaces between the layers in the organic EL panel, are extracted to the outside of the organic EL panel through the transparent substrate 11, and are then visually identified. The emitted light beams 103 and 104 are totally reflected at the interfaces between the transparent substrate 11 and the transparent electrode 14, between the organic EL layer 16 and the cathode 15, and between the transparent substrate 11 and the atmosphere and are not extracted to the outside of the organic EL panel. This is because, for example, in light transmission through the transparent electrode 14, the transparent substrate 11, and the atmosphere in this order, a critical angle due to the differences in refractive index between the layers is generated. For example, the transparent electrode 14 formed of ITO has a refractive index of about 1.9, the transparent substrate 11 formed of glass (for example, soda-lime glass, nonalkali glass) has a refractive index of about 1.5, and the atmosphere has a refractive index of about 1.0. The differences in refractive index are large. The light entered at the critical angle or more is therefore totally reflected at the interfaces between the layers and is repeatedly multiply reflected between each layer and the cathode 15, and cannot be extracted to the atmosphere.
In order to solve this problem, scattering and diffusion of light emitted from the organic EL layer is controlled by providing a layer containing fine particles on the light-transmitting substrate (for example, Patent Documents 1 to 3).